This invention generally relates to the desulfurization of carbonaceous materials containing pyritic sulfur and specifically relates to the desulfurization of coal and solid coal derivatives containing pyritic sulfur through the use of manganese, vanadium, and cerium oxidants in an aqueous acidic medium.
The present use of coal in the United States is primarily for the purpose of conversion into electrical energy and thermal generating plants. A principal drawback in the use of coal on a more widespread basis is its sulfur content, which can range up to five percent. The removal of sulfur from any liquid or solid fossil fuel improves the fuel for use in energy release by oxidation without pollution. Furthermore, the removal of sulfur from coal and solid coal derivatives permits more efficient use of coal in producing liquid fuels and feedstocks, in gasification processes, and in metallurgical processing.
In recent years, air and water pollution resulting from mining and burning of coal has come under public scrutiny. Earlier concern was over the smoke produced from coal-burning installations. Efforts were directed toward more complete combustion in power plants, development of processes for smokeless fuel for domestic use, and reduction of dust effluent from chimneys. More recently, sulfur in coals and rocks overlying coalbeds has received wide attention as a major cause of air and water pollution. In recent years, for example, 209 million tons of coal containing an average of 2.5 percent sulfur was burned in the United States; the sulfur discharged to the atmosphere, mainly as sulfur dioxide, amounted to about 5 million tons. Considering the subsequent increase in power demand which will continue into the foreseeable future, the seriousness of the problem is impressive. Accordingly, both State and Federal Governments have enacted legislation and promulgated regulations which place upper limits on the sulfur content of coals to be burned or on the sulfur dioxide content of the discharged flue gas. However, additional processing of coal, either by processing the coal before it is burned or by processing the flue gas after the coal is burned, adds to the cost of products derived from it -- electricity, for example. Thus, the problem of pollution caused by the combustion of coal or coal-derived fuels affects utilization of coal as a source of power and, hence, its value as a natural resource. Therefore, the cost of removing sulfur from coal must be kept reasonably low, so that coal may be efficiently and economically used as an alternative energy source.
The sulfur in coal occurs in three forms: (1) pyritic sulfur in the form of pyrite or marcasite, (2) organic sulfur, and (3) sulfate sulfur. However, the primary sulfur contaminants are of the first two forms. One solution to the coal desulfurization problem is the removal of sulfur dioxide from flue gas generated by combustion of the coal; another is the removal of sulfur before the coal is combusted or otherwise used. The present invention is a solution of the latter type, and is more specifically described as the chemical removal of inorganic sulfur, especially pyritic sulfur, under relatively mild reaction conditions.